Prime numbers.

A prime number is an integer bigger than zero which can not be expressed as the division of two positive integer numbers with the exception of number 1 and itself.

Indeed, prime numbers can be defined as: "Numbers which can not be expressed as the product of two positive integer numbers smaller than the him."

Notice that number 1 is not included into the prime numbers. 

Let's make an example:

   a)Number 7 only can be divided by 1 and by 7(itself). And It only can be expressed as the multiplication of 7 x 1. In conclusion, as number  7 can no be expressed by either the division of two number different from 1 and itself we can say that 7 is a prime number.

    b) Number 15 can be divided by 1,3,5 and 15, because we can multiply 3x5, divide by 3, divide by 5 ...etc. In conclusion, as number 15 can be expressed by more than 2 number it is not an integer.

The prime term does not mean being relative of someone. It came from the Latin "primus", and it means "the first" (protos in Greek). The fundamental theorem of arithmetic states that every integer is expressed uniquely as a product of prime numbers. That is why they are considered "the first", because from the most primitive ones we are able to get all other integers. (Ex: The number 15 is obtained by multiplying the primes 3 and 5).

Finally, the firsts 25 primes numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89 y 97, which all are smaller than 100.